Quietly exit if there's no Fuchsia SDK for the platform

More suitable for DEPS runhooks running.

Bug: 724204
Change-Id: I105a57269b4c4f7b4b05bab80df7a4ef112bdf1f
Reviewed-on: https://chromium-review.googlesource.com/1217850Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590332}

build/fuchsia/update_sdk.py

@@ -66,6 +66,12 @@ def main():
     print >>sys.stderr, 'usage: %s' % sys.argv[0]
     return 1
 
+  # Quietly exit if there's no SDK support for this platform.
+  try:
+    GetHostOsFromPlatform()
+  except:
+    return 0
+
   # Previously SDK was unpacked in //third_party/fuchsia-sdk instead of
   # //third_party/fuchsia-sdk/sdk . Remove the old files if they are still
   # there.